Diarrhoea is a major health problem worldwide causing morbidity, but also mortality especially of infants in the developing countries. Diarrhoea is the most reported problem for travellers and is commonly caused by contamination of food or water. In most cases traveller's diarrhoea is mild and short of duration, but severe infections with abdominal pain, bloody diarrhoea and septicaemia exist.
The causes of acute diarrhoea of travellers are many and varied. In addition to classical diarrhoeal bacteria, such as Salmonella, Campylobacter, Shigella and Yersinia also diarrhoeal E. coli strains are associated with traveller's diarrhoea. (enterohemorrgenic E. coli; EHEC, enterotoxigenic E. coli; ETEC, attaching and effacing E. coli; A/EEC or enteroaggregative E. coli; EAEC, enteropathogenic E. coli; EPEC, verocytotoxin producing E. coli; VTEC, enterohemorrhagic E. coli; EHEC, enteroinvasive E. coli; EIEC). Salmonella infection can cause a variable clinical disease starting from a mild, subclinical infection, or lead to severe systemic infection, typhoid fever. Salmonella sp. invades the host through the colonic epithelial cells, especially M cells using a type III secretion system. They are also able to survive within phagosomes of macrophages, and evade the host immune system by several ways (Coburn et al., 2007). Campylobacter jejuni and coli are among the large Campylobacter family predominant human stool pathogens causing watery diarrhoea, fever and typically hard abdominal pain. By diagnostic means they must be dissected from the other Campylobacter species not associated with diarrhoea. They are able to invade the colonic epithelium lining and replicate intracellularly and cause apoptosis (Poly and Guerry, 2008; Allos, 2001). Yersinia enterocolitica and pseudotuberculosis harbour a virulence plasmid containing relevant adhesion and invasion proteins, such as YadA (Bottone, 1999, El Tahir et al., 2001). For Y. enterocolitica a virulence plasmid is required to cause a clinical disease, whereas Y. pseudotuberculosis has additional genomic virulence factors as well. Yersinia pestis is the plague pathogen, which harbours genomic virulence factors and three virulence plasmids which are all required to cause a clinical disease (Bottone, 1999). The traditional pathogens are also associated with late onset symptoms, such as reactive arthritis, sacroiliitis and acute anterior uveitis.
Vibrio cholerae is a highly virulent environmental pathogen living in free waters in some of the tropical countries, especially causing epidemics in catastrophe areas. It typically causes massive watery diarrhoea leading to patient death if not sufficiently resuscitated. The essential virulence factor is cholera toxin which consists of two subunits A and B. The cholera toxin is able to bind irreversibly to the G-proteins in the colonic epithelial cells responsible for liquid and electrolyte uptake causing non-voluntary continuous secretion into gut lumen (Nelson et al., 2009).
Shigella and EIEC are genetically closely related. Both of these organisms invade the colonic epithelium mediated by the genes located in virulence plasmid pINV coding e.g. Ipa proteins and their transcription regulator invE (Lan and Reeves, 2002; Parsot, 2005). EAEC demonstrate characteristic adherence pattern to Hep-2 cells via specific fimbria encoded by genes which are located on plasmid under the regulation of AggR (Flores and Okhuysen, 2009). EPEC is characterized to possess pathogenicity island named the locus of enterocyte effacement (LEE). This island contains genes such as eae for intimate adherence of the EPEC strains to intestinal epithelial cells. EPEC is differentiated from EHEC by ability of EHEC strains to production shiga-like toxins I and II encoded by stx1 and stx2 genes. These cytotoxins cause acute inflammation in the intestine leading to abdominal pain and bloody diarrhoea. In addition, EHEC infection may lead to rare but severe, secondary complications such as haemolytic uremic syndrome (HUS) (Chen and Frankel, 2005; Karch et al., 2005). The challenge in multiplex PCR assays is to identify EHEC variants so that there is no cross-reaction with Shigella/EIEC species, because the target genes expressing toxins in these bacteria are very similar.
Giardiasis is an infection of the small intestine caused by Giardia lamblia (also known as G. intestinalis), a flagellate protozoan. Giardiasis is the most commonly reported pathogenic protozoan disease worldwide. Travelers are the largest risk group for giardiasis infection, especially those who travel to the developing world. Giardiasis is spread via the fecal-oral route. Most people contract the disease by ingesting contaminated water or food, or by not washing their hands after touching something contaminated with Giardia cysts. Prevalence rates for giardiasis range from 2-7% in developed countries and 20-30% in most developing countries. The CDC estimates there are an upwards of 2.5 million cases of giardiasis annually. The most common symptoms of Giardia infection include diarrhea for a duration of more than 10 days, abdominal pain, flatulence, bloating, vomiting, and weight loss. Giardiasis is traditionally diagnosed by the detection of cysts or trophozoites in the feces, trophozoites in the small intestine, or by the detection of Giardia antigens in the feces.
Currently, the routine diagnostic of diarrhoea is mostly based on traditional cultivation methods and immunoassays, which are both laborious and time consuming. They are only available for Salmonella, Campylobacter, Shigella and Yersinia species as well as enterohaemorrhagic Escherichia coli (EHEC), whereas no cultivation method for other major diarrhoeagenic E. coli species, including ETEC, EPEC, EAEC, and EIEC exists. In recent years, DNA based methods for diagnosis of diarrhoeagenic E. coli has been published (Antikainen et al., 2009; Aranda et al., 2004; Brandal et al., 2007; Guion et al., 2008; Kimata et al., 2005; Müller et al., 2007; Vidal et al., 2005; Vidal et al., 2004).
ETEC causes watery diarrhoea by producing heat-labile (LT) and/or heat-stable (ST) enterotoxins [2-3]. ETEC is traditionally considered the most common cause in traveller's diarrhoea (Qadri et al., 2005). The present invention is particularly directed to improve the detection of ETEC in multiplex RT-PCR assays. The present invention provides two primer pairs and probes specific for the heat stable enterotoxin of ETEC encoded by the est gene and one primer pair and probe specific for the heat labile enterotoxin of ETEC encoded by the elt gene. These primers and probes are designed to amplify such target sequences in said genes that it renders possible efficient detection of global variants of ETEC. Further problem was that the target gene est includes multiple repetitive elements and it was difficult to find conserved regions long enough for both the primers and the probe.
The present invention is further directed to improve the detection of diarrhea causing Campylobacter species in multiplex RT-PCR assays. The invention provides primer pairs and probes for rimM gene of C. jejuni and gyrB gene of C. coli. With these primers and probes the diarrhoea causing Campylobacter can be distinctively identified from other non-pathogenic Campylobacter and other diarrhea causing pathogens. A combination of two different genomic targets was required to solve the problem.
In WO2005/005659, it is disclosed a method for simultaneous screening diarrhoea causing bacteria such as E. coli groups: ETEC (enterotoxigenic E. coli), A/EEC (attaching and effacing E. coli) EPEC (enteropathogenic E. coli), VTEC (verocytotoxin producing E. coli) and EIEC (enteroinvasive E. coli); and Shigella spp. The method is a real-time multiplex PCR assay and the template DNA is isolated directly from a stool sample. Similarly as the present invention, WO2005/005659 is also directed to the problem of screening for human pathogenic E. coli in order to provide distinction between the pathogenic E. coli groups and other diarrhoea causing pathogens. However, the target sequences in est and elt genes of ETEC are different in the present invention from the targets disclosed by WO2005/005659. Moreover, the present invention is providing coverage of global ETEC variants by the use of three specific primer pairs and probes while WO2005/005659 in Table 3 discloses four primer pairs for the same purpose. Table 8 of the present specification show that ETEC variants can be detected by using the three primer pairs of the present invention.
In WO2005/083122, it is disclosed a method for detection and quantification of enteropathogenic bacteria in a fecal specimen, including Shigella species, Salmonella species, Campylobacter species, enterohemorrhagic Escherichia coli or Verocytotoxin-producing Escherichia coli, Vibrio cholerae, and Clostridium perfringens. The method is a real-time PCR assay based on TaqMan® probes.
In WO2007/056463, it is disclosed a method comprising amplification of a sample with a plurality of pathogen-specific primer pairs to generate amplicons of distinct sizes from each of the pathogen specific primer pairs. The method utilizes real-time and multiplex PCR techniques. The method can be used for the detection of Salmonella species, Campylobacter species, diarrhoeagenic Escherichia coli, Vibrio cholerae, Yersinia species such as Yersinia pestis, and Giardia lamblia. 
In WO2005/090596, it is disclosed an assay for detecting micro-organisms, and in particular bacteria, based on multigenotypic testing of bacterial DNA from human, animal or environmental samples. The method may also be utilized as a real-time multiplex PCR technique using TaqMan® probes. The method can be used for the detection of Salmonella species, Campylobacter species, diarrhoeagenic Escherichia coli, Vibrio cholerae, Yersinia species such as Yersinia pestis. 
In US2004/0248148, it is disclosed a 5′ nuclease real-time polymerase chain reaction approach for the quantification of total coliforms, E. coli, toxigenic E. coli O157:H7, toxigenic M. aeruginosa (microcystin hepatotoxins), Giardia lamblia, and Cryptosporidium parvum. Multiplex PCR assay can also be used for simultaneous detection of two or more pathogens.
In WO02/070728, it is disclosed an assay that relies on a ‘multiprobe’ design in which a single set of highly conserved sequences encoded by the 16S rRNA gene serves as the primer pair, and it is used in combination with both an internal highly conserved sequence, the universal probe, and an internal variable region, the species-specific probe. The real-time system reliably identifies 14 common bacterial species.
CN101113471, CN101245384 and CN101235410 disclose PCR methods for rapid detection of diarrhoea causing pathogens from food samples.
Fukushima et al., 2003, disclose a real-time PCR assay for detection of 17 species of food- or waterborne pathogens directly from stool sample. The detection levels were approximately 105 pathogenic bacteria per gram of stool, therefore the protocol for stool specimens for less than 104 pathogenic bacteria per gram of stool requires an overnight enrichment step to achieve adequate sensitivity.
Hidaka et al., 2009, disclose multiplex real-time PCR for exhaustive detection of diarrhoeagenic E. coli. This method is especially for the detection of pathogenic bacteria from a food sample, such as meat sample.
Wang et al., 1997, disclose a protocol for PCR detection of 13 species of foodborne pathogens in foods.
There are some commercial multiplex PCR-based diarrheal pathogen detection kits available, for example xTAP GPP from Luminex and Diarrhea ACE Detection from Seegene. Both systems use multiplex PCR as a means for amplifying certain organism-specific nucleotide sequences but the final detection relies on analysis in another separate instrument. The means used for detection has an impact on the amplicon, primer and probe design because of different requirements of the detection formats. Further, gene or amplicon sequences used in the present invention for the detection of ETEC or Campylobacter have not been disclosed.
The number of pathogens causing diarrhoea is large and a diarrhoea test method should optimally identify all of them. Having one PCR reaction per species can be cumbersome, since the number of samples tested is typically large. It would be optimal to detect multiple species within one reaction. In a PCR setting the most obvious alternative is ‘multiplex’ PCR amplification. In multiplex PCR, several oligonucleotide sets, each designed to amplify one species/species group, are included in the same reaction vessel and each oligonucleotide set is used to amplify its respective pathogen DNA during the same PCR reaction. In this invention, we describe a PCR based method for rapid detection of clinically important pathogens related to traveller's diarrhea, particularly ETEC and/or Campylobacter. The present invention discloses primers and probes designed for target sequences conserved in global variants of ETEC and Campylobacter. These primers and probes are compatible for use in any multiplex RT-PCR determining the presence of multiple diarrhoea causing pathogens, since the target sites are unique for ETEC and Campylobacter. 
Multiplex PCR presents a challenge for quantitation of the pathogen DNA (qPCR): the different amplicons compete for the same PCR reaction components (eg. DNA polymerase and MgCl2) and this can compromise the quantitative nature of the reaction between and, especially, quantitative comparisons between samples. It is commonly known in the art that there is bias in the amplification efficiencies between different template amounts or lengths so that e.g. short amplicons are favoured in the expense of longer ones.
At the same time, undesired cross-reactions of multiplex set oligo combinations must be avoided. One must also remember to check mis-priming to any other sequences present in the sample.
Finding suitable primer and probe sequences for the detection of a diverse group of pathogenic microbes can be far from trivial especially when designing multiplex set ups where all amplicons and templates should be amplified with equal efficiency (e.g., Giardia). Many of the species are relatively closely related, making it challenging to locate sequences that are unique for each species. Also, as there are a significant number of global variants, it is difficult to identify globally conserved regions or a combination of minimal set of regions to detect all known variants (e.g., for EHEC, ETEC, pathogenic Yersinia, pathogenic Campylobacter and Shigella/EIEC). Some genes possess complex repetive closely related elements which is challenging from the amplicon design point of view, especially when designing amplicons for multiplex PCR. For example, due to repetitive elements and minor variants ETEC cannot be detected using only one amplicon.
The sample matrix, which in diarrhoea diagnostics is commonly a stool or food sample, is likely to contain a host of PCR inhibitors. This reduces amplification efficiency of the PCR reaction and thus even more careful optimization is expected from the amplicon design step to verify that all templates and copy numbers are amplified equally but also efficiently enough. Hence, oligonucleotide design enabling high PCR efficiency (optimally as close to 100% as possible) is required. The detection method used may also affect amplification efficiency and/or bias.
The present inventors have now located DNA sequence regions that are well suited for specific and sensitive amplification and quantification of diarrhoea causing pathogens, particularly ETEC and Campylobacter. Accordingly, optimal primers and quantitative PCR probes have been designed in the present invention and validated for identification and quantification of diarrhoea causing pathogens. The amplicons have been designed to be so specific that they can be combined into any multiplex sets with each other. Naturally a prerequisite to this is that all the disclosed amplicons have also been designed to amplify in the same reaction and cycling conditions.